1. Field of the Invention
The present invention relates to a laser light multiplexing apparatus that causes an entire bundle of light beams emitted from a plurality of semiconductor lasers to converge and enter an optical fiber.
2. Description of the Related Art
U.S. Patent Laid-Open No. 20020090172 discloses a method of propagating laser light within an optical fiber. In this method, laser light beams emitted from a plurality of semiconductor lasers arranged in a single direction are collimated through collimator lenses so as to obtain collimated light beams. The optical axes of the collimated light beams are parallel to each other and aligned along a single direction. All of the collimated light beams are collected and are individually condensed so that the collected and condensed light beams enter a single optical fiber. Thereby, laser light propagates through the optical fiber with high energy density.
It is known that the entirety of a plurality of light beams, the optical axes of which are aligned along a single direction, can be converged into a small cross section when the fast axes of the light beams are aligned along a single direction. That is, light beams emitted from each semiconductor laser have fast axes in the thickness direction of the active layer of the semiconductor laser. The light beams also have slow axes in a direction perpendicular to the thickness direction and parallel to the surface of the active layer of the semiconductor laser. The above light beams have satisfactory wavefronts in the directions of the fast axes and can be accurately converged. However, satisfactory wavefronts are not formed in the directions of the slow axes. Therefore, it is not possible to accurately converge the above light beams in the directions of the slow axes.
In another known method for generating a plurality of laser beams, a plurality of semiconductor lasers are arranged on a single substrate. In this case, the surfaces of the active layers of the semiconductor lasers are formed parallel to the surface of the substrate. Therefore, the slow axes of the light beams emitted from the plurality of semiconductor lasers arranged on the single substrate are coplanar. Therefore, in order to optically multiplex a plurality of light beams into a single optical fiber with high coupling efficiency by using the above substrate, it is necessary to realign the above light beams so that the fast axes of the light beams are aligned in a plane, before the entirety of light beams are converged. U.S. Pat. Nos. 5,513,201, 5,808,323, and 6,028,722 propose methods for rearranging the above light beams in the fast and slow axes. Hereinafter, the rearrangement of light beams in the fast and slow axes will be referred to as light beam rearrangement. Note that the aforementioned coupling efficiency is light utilization efficiency during optically multiplexing of light beams.
In the technique disclosed in U.S. Pat. No. 5,513,201, the above light beam rearrangement is realized by providing prisms in correspondence with each of the light beams and arranged along a direction perpendicular to the propagation direction of the light beams. In this case, it is necessary to spread the intervals between the light beams, to match the arrangement of the prisms. This is because it is difficult to cause the light beams to enter the prisms in a state in which the intervals therebetween are small. Therefore, the size of an apparatus, including a substrate on which semiconductor lasers having wide intervals therebetween, is increased. In addition, there is a problem that spatial utilization efficiency (to be described later) is decreased, leading to a deterioration of the aforementioned coupling efficiency of the light beams and an optical fiber.
If laser light is to be efficiently utilized, it is common practice to administer highly reflective coatings on reflecting surfaces for reflecting the laser light. It is difficult to administer highly reflective coatings on the great number of complexly shaped reflecting surfaces disclosed in U.S. Pat. No. 5,808,323. Similarly, it is difficult to administer highly reflective coatings on the complexly shaped prisms disclosed in U.S. Pat. Nos. 5,513,201 and 6,028,722. However, the amount of loss of the laser light is high along the propagating optical path thereof where the highly reflective coating is not administered, which leads to deterioration of the utilization efficiency. In the case that light beams are reflected a great number of times (five times or greater) within a prism as disclosed in U.S. Pat. No. 6,028,722, the amount of loss of the laser light is particularly high.
Hereinbelow, the space utilization efficiency will be explained in detail with reference to FIGS. 28A, 28B, 28C, 29A, 29B, and 30. FIGS. 28A, 28B, and 28C show a schematic construction of a conventional laser light multiplexing apparatus. FIG. 28A is a plan view, FIG. 28B is a side view from the direction along which semiconductor lasers are arranged, and FIG. 28C is a view from the direction of the optical axes of light beams. A collimating optical system, which is arranged between the semiconductor lasers and the light beam rearrangement optical system, is omitted from FIG. 28C. FIGS. 29A and 29B are provided to illustrate rearrangement of the light beams and optical multiplexing in the laser light multiplexing apparatus. FIG. 29A shows a light beam rearrangement optical system rearranging the directions of axes of the light beams, and FIG. 29B shows optical multiplexing of the light beams in an optical fiber. FIG. 30 is a perspective view illustrating arrangement of prisms constituting the light beam rearrangement optical system.
The laser light multiplexing apparatus illustrated in FIGS. 28A, 28B, and 28C includes a laser block 70, the collimating optical system 75, the light beam rearrangement optical system 80, and a convergence optical system 85. Five semiconductor lasers 71A, 71B, 71C, . . . are provided on the laser block 70 in such a manner that active layers 72A, 72B, 72C, . . . of the semiconductor lasers 71A, 71B, 1C, . . . are coplanar and are aligned along the Y direction indicated in FIGS. 28A, 28B, and 28C. The collimating optical system 75 collimate light beams La, Lb, Lc, . . . which are emitted from the semiconductor lasers 71A, 71B, 71C, . . . in the Z direction indicated in FIGS. 28A, 28B. The collimated light beams La, Lb, Lc, . . . are parallel to each other, and have coplanar slow axes. The light beam rearrangement optical system 80 is constituted by five prisms 81A, 81B, 81C, . . . arranged along a direction perpendicular to the propagation direction of the light beams La, Lb, Lc, . . . in correspondence with the light beams. The light beam rearrangement optical system 80 rearranges the light beams collimated by the collimating optical system 75. The convergence optical system 85 converges the entire bundle of the light beams rearranged by the light beam rearrangement optical system 80, in the directions of the slow and fast axes.
The collimating optical system 75 is constituted by collimator lenses 76A, 76B, 76C, . . . .
The light beam rearrangement optical system 80 changes the directions of the fast axes of the light beams La, Lb, Lc, . . . collimated by the collimating optical system 75, from the X directions to the Y directions so that the fast axes of the light beams are coplanar.
That is, in the laser light multiplexing apparatus illustrated in FIGS. 28A, 28B, and 28C, the light beams La, Lb, Lc, . . . emitted from the semiconductor lasers 71A, 71B, 71C, . . . are collimated by the collimating optical system 75 into collimated light beams with parallel optical axes and coplanar slow axes. Then, the collimated light beams La, Lb, Lc, . . . pass through the prisms 81A, 81B, 81C, . . . , and rearranged so that the fast axes of the collimated light beams La, Lb, Lc, . . . are oriented in the Y direction and are coplanar (as illustrated in FIG. 29A). The entire bundle of the light beams rearranged as above are converged so that the widths of the entire bundle, in the directions of the fast and slow axes (F and S, respectively), are reduced, and enters a core 41 of an optical fiber 40 (as illustrated in FIG. 29B).
Light beams are converged with higher quality in the directions of the fast axes than in the directions of the slow axes. Therefore, the rearranged light beams having the coplanar fast axes can be coupled to the core 41 of the optical fiber 40 with high coupling efficiency.
In the case where each of the prisms 81A, 81B, 81C, . . . is formed by combining prism portions J1, J2, and J3 each having a shape of a triangular prism as illustrated in FIG. 30, each of the light beams enters the prism portion J1 in one of the prisms 81A, 81B, 81C, . . . corresponding to the light beam, and is reflected in the prism portion J1 so as to be redirected to the Y direction. Subsequently, the light beam reflected in the prism portion J1 is further reflected and redirected in the prism portions J2 and J3, and is thereafter output from the prism portion J3. However, as illustrated in FIG. 30, it is necessary to provide spacing between adjacent prism portions J3 for placing the prism portions J1. That is, it is impossible to arrange the adjacent prism portions J3 close to each other. Therefore, there are substantial gaps G between the light beams La, Lb, Lc, . . . converged by the convergence optical system 85 as illustrated in FIG. 28A.
When the focal length and the numerical aperture of each of the collimator lenses 76A, 76B, 76C, . . . are indicated by f1 and NA1, the focal length of the convergence optical system 85 is indicated by f2, the numerical aperture of the optical fiber 40 indicated by NA2, and the space utilization efficiency indicated by η, the magnification power M of the lens system, i.e., the ratio of the dimension of the light emission spot (the active layer of each of the semiconductor lasers 71A, 71B, 71C, . . . ) to the dimension of the convergent spot which each of the light beams La, Lb, Lc, . . . forms on an end face of the core 41 of the optical fiber 40, is expressed by the following equation (1).                     M        =                                            f              2                                      f              1                                =                                                    NA                1                                            (                                                                            NA                      2                                        N                                    ×                  η                                )                                      =                                                            NA                  1                                                  NA                  2                                            ×                              N                η                                                                        (        1        )            Note that N denotes the number of the light beams to be optically multiplexed. In addition, the space utilization efficiency η is defined as a ratio of the sum of the spaces occupied by the respective light beams La, Lb, Lc, . . . to the continuous space containing the entire bundle of the light beams La, Lb, Lc, . . . (i.e., the space between and including the light beams La and Le) . Therefore, when the optical paths of the seven laser beams La through Le are adjacent each other, η=1.
As clearly shown by equation (1), the magnification power M decreases with an increase in the space utilization efficiency η. In addition, the displacement of each of the light beams La, Lb, Lc, . . . on the end face of the core 41 of the optical fiber 40 caused by misalignment between the relative positions of the semiconductor lasers 71A, 71B, 71C, . . . , the convergence optical system 85, and the optical fiber 40 decreases with a decrease of the magnification power M. Therefore, the accuracy in the optical multiplexing of the light beams increases as the magnification power M decreases.
When there are substantial gaps between the light beams rearranged by the light beam rearrangement optical system, the space utilization efficiency η is small. Therefore, in this case, it is difficult to accurately perform the optical multiplexing of the light beams after the entire bundle of the light beams is converged, and thus the coupling efficiency of the light beams to the optical fiber decreases.